The PWM is a traditional technology used in the switching mode power converter to control the output power and achieve the regulation. Most of the equipments, such as mobile phone, TV game, computer and so on are using PWM power converters to supply power and charge battery. Various protection functions such as over-voltage and over-current protection are built in the power supply to protect the power supply and the load from permanent damage. The function of output power limit is generally used for the over-load and short circuit protection.
Referring to FIG. 1, it shows a conventional application circuit of the PWM power supply. A PWM controller 10 controls the power output and achieves the regulation. The operation of PWM-control starts on the charging of a start-up capacitor 18 via a serial start-up resistor 12 when the power is turned on until the supply voltage VCC reaches the threshold voltage, and then the PWM controller 10 starts to output a PWM signal and drive the entire power supply. After the start-up, the supply voltage VCC is provided from the auxiliary bias winding of the transformer 20 through a rectifier 19. A resistor 11 that is connected serially with the power metal-oxide semiconductor field-effect transistor (MOSFET) 17 determines the maximum output power of the power supply. The method is to connect the voltage (VS) of resistor 11 to the current-sense input of the PWM controller 10. If the voltage VS is greater than the maximum current-sense voltage such as 1V, the PWM controller 10 will disable the output of its OUT pin and restrict the maximum power output of the power supply. The energy stored in an inductor is given by
                    E        =                                            1              2                        ×                          L              p                        ×                          I              p              2                                =                      P            ×            T                                              (        1        )            
Where Ip and Lp are the peak current and the primary inductance of the transformer 20 respectively, and T is the PWM switching period. The peak current Ip can be expressed as follows:
                              I          p                =                              Vin                          L              p                                ×                      t            ON                                              (        2        )            
Where tON is the turn-on time of the PWM signal in which the power MOSFET 17 is switched on, and Vin is the input line voltage. Thus the output power (P) of a PWM power supply can be calculated as follows:
                    P        =                                                            Vin                2                            ×                              t                ON                2                                                    2              ×              T              ×                              L                p                                              ×          η                                    (        3        )            
Where η is the energy transfer ratio from primary to secondary. Assuming the load is constant, as Vin varies from low to high, the output voltage (Vout) will be kept constant by automatically adjusting the tON through the feedback control loop of the power supply, and tON can be calculated from the equation (4) and the equation (5).
                    Vin        =                ⁢                                            N              ×                              (                                  1                  -                  D                                )                                      D                    ×          Vout                                    (        4        )                                D        =                ⁢                              t            ON                    T                                    (        5        )            
Where N is the turn-on ratio, but the maximum tON is restricted and can be expressed as the equation (6) when the voltage in the VS pin is higher than a power limit voltage Vlimit, such as 1V in the PWM-control IC UC384X.
                              t          ON                =                                            V              limit                        ×                          L              p                                            Vin            ×            Rs                                              (        6        )            
Rs is the resistance of a current sense resistor 11 which is added between the source of the power MOSFET 17 and the ground for current sensing. Furthermore, the maximum output power (Pmax) is also affected by the PWM controller's 10 response time tD. From the moment that the voltage in the VS pin is higher than the power limit voltage to the moment that the PWM controller's 10 OUT pin is actually off, there is a delay time tD. Within this delay time tD, the power MOSFET 17 is still on, and it will continue delivering power. Therefore, the actual turn-on time of the PWM signal is equal to tON+tD, and the actual maximum output power (Pmax) becomes as follows:
                              P          max                =                                            V              in              2                        ×                                          (                                                      t                    ON                                    +                                      t                    D                                                  )                            2                                            2            ×                          L              p                        ×            T                                              (        7        )            
Although the tD time is short, generally within the range of 150˜200 ns, the higher the operating frequency is, the more impact is caused by tD because the switching period T is short and tD becomes relatively more important. Normally, tD is a constant time determined by the controller's speed. Thus, the maximum output power Pmax from the equation (7) will vary as the input line voltage Vin varies. When the safety regulations are taken into consideration, the range of the input line voltage Vin is from 90Vac to 264Vac, wherein the output power limit (PHight) of the power supply in high line voltage (Hight) is many times higher than the output power limit (PLow) in low line voltage (Low). If power limit voltage Vlimit is constant, the max output power Pmax will vary with the line voltage, as shown in FIG. 2. That's why line compensation is needed. And, a high voltage across the resistor 13 causes inconvenience for the component selection and printed circuit board (PCB) layout.